Music VideosWilliam Wieland
The Music Animation Machine has made possible many wonderful videos. A selection follows:
Bach, Preludio, Partita in E Major, Lara St. John, solo violin (3:09) — You can clearly see shapes and patterns created by the musical texture.
Bach, Cello suite 1, 1st mvt. ("caterpillar" animation) (3:03) — The colored dots show the implied polyphony of this monophonic composition.
Chopin, Nocturne in E-flat Major, opus 9 no.2, Piano Solo (animated score) (3:58) — Each voice (melody, bass and inner parts) is clearly visible.
Tarrega, Recuerdos de la Alhambra, guitar, James Edwards (ver. 1) (3:58) — Each voice is once again clearly visible.
Franck, Violin Sonata (6:35) — You can see the piano and violin canon. The piano's right hand leads in octaves and the violin follows yet an octave higher.
Beethoven, Symphony 9, 2nd movement (complete), Molto vivace, Philharmonia Baroque (13:34) — What color is each instrument?
Bohemian Rhapsody Played by 100+ year old fairground organ (5:29) — "Old School" instrument which uses a similar graphic score

Many videos couple a fantastic audio recording with a score. You don't even need to turn pages!
Palestrina - Sicut cervus - The Cambridge Singers (3:17) — As an ear training exercise, follow and listen to one part as the video plays.
Mozart-Ave Verum Corpus-Alto-Score.wmv (2:24) — Many videos highlight one part.

Playing For Change — "Connecting the World through Music"
Stand By Me | Playing For Change | Song Around the World (5:28) — Perhaps their most famous video
What a Wonderful World | Playing For Change (3:41) — Children's Choirs with Grandpa Elliott

Eric Whitacre has led several Internet performances of his original choral music.
Eric Whitacre's Virtual Choir - 'Lux Aurumque' (6:20) — 2010
Eric Whitacre - Eric Whitacre's Virtual Choir 2.0, 'Sleep' (9:31) — 2011
Eric Whitacre: A virtual choir 2,000 voices strong (14:35) — a TED video from 2011
Eric Whitacre's Virtual Choir 3, 'Water Night' (11:43) — 2012

Random Acts of Culture are fantastic, e.g. Opera Company of Philadelphia "Hallelujah!" Random Act of Culture (5:59).
NPR Tiny Desk Concerts are also fantastic, e.g. Cantus: Tiny Desk Concert (15:57) — Minnesota male a cappella ensemble

Apropos and Momentous Performances
4th Movement - part 1 - Ode to Freedom - 1989 - Leonard Bernstein - Beethoven's 9th Symphony HD 720p (12:01) — performed
  in celebration of the fall of the Berlin Wall, Bernstein changed Beethoven's "Ode to Joy" (Freude) to "Ode to Freedom" (Freiheit)
BARBER - ADAGIO FOR STRINGS - 9/11 TRIBUTE (11:16) — a very sad piece performed by the BBC Orchestra under the direction of
  Leonard Slatkin (American) and broadcast across Britain on September 15, 2001 in honor of those who lost their lives a few days prior

10000 singing Beethoven - Ode an die Freude _ Ode to Joy (18:53) — I thought the South Dakota All-State Choir was large!
ZKO Rollercoaster (1:00) — simply fun
Line Rider - Mountain King (2:45) — more fun
Beethoven 5.1 (7:17) — The first movement of Beethoven’s 5th Symphony performed by the Gas House Gang, a barbershop quartet!

Northern Cree Crow Hop @ Mandaree 2015 (3:03)
Supaman – 'Prayer Loop Song' (4:03)
I could go on forever...